1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding sheets, such as originals and recording paper, to an image forming apparatus, such as a copying machine, a printer, or a facsimile apparatus.
2. Related Background Art
Some conventional copying machines or the like are equipped with a retard separation type automatic sheet feeding apparatus in which sheets serving as recording paper and are accommodated in a sheet cassette or the like are sent out one by one through a feed roller rotating in the sheet feeding direction and a retard roller capable of reverse rotation. An example of such a conventional sheet feeding apparatus will be described with reference to drawings. FIGS. 4 through 6 are diagrams showing a retard separation type sheet feeding apparatus, of which FIG. 4 is an explanatory sectional view of a sheet feeding means, FIG. 5 is a perspective view showing a drive transmission portion for driving the sheet feeding means, and FIG. 6 shows a main portion of the sheet feeding means.
The sheet feeding means 1 is equipped with a deck 5 for accommodating sheets S, a pick-up roller 2 for sending out sheets S from the deck 5, and a pair of sheet feeding rollers 3 and 4, which is made up of a feed roller 3 and a retard roller 4. Reference numeral 15 indicates a torque limiter for transmitting a torque of a predetermined torque value or less. The feed roller 3 is normally caused to rotate in the sheet feeding direction (the direction indicated by the arrow A in FIG. 5) through a feed roller shaft 11, and a rotational force in a direction opposite to the sheet feeding direction (i.e., the direction indicated by the arrow B in FIG. 5) is transmitted to the retard roller 4 through a retard roller shaft 13 and the torque limiter 15. The sheets sent out by the pick-up roller 2 are guided to the pair of sheet feeding rollers 3 and 4 by guides 7 and 8, and are separated from each other by the pair of sheet rollers 3 and 4 to be conveyed by a conveying roller pair 6 while being guided by a guide 9.
When the pick-up roller 2 feeds one sheet S from the deck 5, the torque limiter 15 makes idle rotation due to the frictional force between the sheet S and the feed roller 3, and the rotational force of the retard roller 4 in the direction of the arrow B is interrupted. Thus, the retard roller 4 follows the feed roller 3 to rotate therewith, thus feeding the sheet S.
When the pick-up roller 2 feeds a plurality of sheets S, the frictional force between the sheets S is smaller than the frictional force between the sheets S and the feed roller 3, so that the retard roller 4 rotates in the direction of the arrow B to restore the sheets to the interior of the deck 5 except for the uppermost sheet.
In FIG. 6, reference numeral 102 indicates a pressurizing arm, which is mounted on a support member 100 fixed to the apparatus main body and rotates around a pivot 102A. Reference numeral 101 indicates a spring. A pressurizing force due to the spring 101 causing the pressurizing arm to rotate around the pivot 102A is applied to the retard roller 4, providing a contact pressure for the feed roller 3. Together with the idle rotation torque T of the torque limiter 15, this contact pressure constitutes an important parameter determining “double feeding” and “slippage” of the sheets being fed. In the following, the phenomenon in which a plurality of sheets are fed simultaneously into the main body of the image forming apparatus due to the failure of the pair of sheet feeding rollers 3 and 4 to effect sheet separation, will be referred to as “double feeding”, and the phenomenon in which the sheets are not fed beyond the pair of sheet feeding rollers 3 and 4 will be referred to as “slippage”.
FIG. 7 is an explanatory diagram illustrating a feeding area involving no double feeding or slippage. In the drawing, the horizontal axis indicates the idle rotation torque T of the torque limiter 15, and the vertical axis indicates the pressurizing force N (contact pressure) of the retard roller 4, with the shaded portion indicating the feeding area where separate feeding of the sheets is possible.
Thus, assuming that, a predetermined value T1 is set in FIG. 7 for the idle rotation torque of the torque limiter 15 in order to prevent “double feeding” and “slippage”, it can be understood that the contact pressure is restricted to the range as defined by the resultant intersections N1 and N2 and the border lines (3) and (5). Thus, the value of the contact pressure is a very important parameter in determining the feeding area. Further, this contact pressure fluctuates upon drive input to the pair of sheet feeding rollers 3 and 4.
Here, the theory on the fluctuation in the contact pressure of the retard roller 4 for the feed roller 3 will be described in detail. First, in FIG. 5, the arrow a indicates the direction in which sheets are sent out. Due to the rotational force of the retard roller 4 in the direction of the arrow B, a return force due to the torque limiter 15 (in the direction opposite to the arrow a) is applied to the feed roller 3. As shown in FIG. 6, as the reaction force for this return force, at the point on the outer peripheral surface of the retard roller 4 in contact with the feed roller 3, a force F1 acts in the sheet feeding direction. The forces caused to act by this force F1 are mentioned below. It is assumed here that the point of action of the force applied to the retard roller 4 during drive is not on the outer peripheral surface of the rotating roller but at the roller center.    F1: the reaction force of the return force of the retard roller 4 (=T/r)    F2: the offset force of the force F1    F3: the tangential component of F2 around the center of pivotal movement of the rotating arm 102    Na: the component of F3 directed to the center of the feed roller 3Na=(T/2r)sin 2θ  (1)where    T: the idle rotation torque of the torque limiter;    r: the effective radius of the of the retard roller (which is defined as the actual distance from the retard roller center to the outer peripheral surface of the feed roller); and    θ: the angle of pivot of the pressurizing arm 102 as measured from the line of action of F2.
The action force Na is defined as the fluctuation pressure Na of the retard roller 4. Thus, under static state, it is related to the contact pressure (static pressure) as follows (as shown in FIG. 8):Contact pressure under dynamic state (dynamic pressure)=contact pressure under static state (static pressure)+fluctuation pressure Na  (2)
That is, at the time of drive input, the contact pressure of the retard roller 4 for the feed roller 3 fluctuates due to this fluctuation pressure Na. Here, it is assumed that, in FIG. 6, the angle made by F2 and the line of action connecting the center of the retard roller 4 and the pivotal movement center of the pressurizing arm 102 is θ. In the graph of FIG. 9, the horizontal axis indicates the angle θ, and the vertical axis indicates the fluctuation pressure Na. The graph shows a theoretical value based on equation (1) Here, it is assumed that both the idling torque T of the torque limiter and the effective radius r of the retard roller 4 are fixed.
It can be seen from FIG. 9 that the fluctuation pressure Na can assume a positive or negative value depending upon the angle of pivot θ. Note that, in FIG. 9, when the sign of θ is positive, the sign of Na is also positive.
The relationship expressed by equation (1) and shown in FIG. 9 is restricted to a theoretical value when the effective radius r of the retard roller is fixed. Thus, when the effective radius r of the retard roller 4 is not fixed, that is, when the roller radius can be greatly changed by the pressurizing force applied to the roller surface, as in the case of a roller formed of a sponge or the like or a hollow roller, the relationship is to be indicated by a different curve.
In the graph of FIG. 10, the horizontal axis indicates the angle θ, and the vertical axis indicates the fluctuation pressure Na, showing the measurement results when the material of the retard roller 4 is a sponge. It can be seen from the graph that, as compared with the case in which r is fixed, the degree of inclination of equation (1) is smaller. Apart from this, there may be a case in which the retard roller consists of a rubber roller or the like. In this case, however, due to the small change amount of the effective radius of the rubber roller, the resultant measurement result substantially coincides with the theoretical value in the case shown in FIG. 9, in which the effective radius r is fixed.
The reason why the degree of inclination of the measurement result in FIG. 10 is smaller than that of the theoretical value will be explained. First, upon drive input to the pair of sheet feeding rollers 3 and 4, a fluctuation pressure Na is generated in the contact pressure of the retard roller 4 for the feed roller 3. When the retard roller 4 is formed of a soft material, the roller is crushed by this fluctuation pressure Na, and the center position of the retard roller 4 is displaced, resulting in a change in the effective radius r of the retard roller 4. Assuming that the change amount in this effective diameter r is Δr, the sign of Δr is reverse to that of the fluctuation pressure Na. Here, the spring 101 causes the pressurizing arm 102 to rotate around the pivot 102A to thereby apply a pressurizing force to the retard roller 4, thus providing a contact pressure for the feed roller 3. When the effective radius r of the retard roller 4 is changed by Δr, the displacement amount of the spring 101 is also changed, thus changing the contact pressure for the feed roller 3a as well. Assuming that the change amount in contact pressure due to the change in the displacement amount of the spring 101 is ΔNa, Δr and ΔNa are in the following relationship:ΔNa≈k·Δr  (3)where    k: the elastic modulus of the spring 101
Since the sign of Δr is reverse to that of the fluctuation pressure Na, ΔNa works so as to cancel the fluctuation pressure Na. That is, this ΔNa constitutes a factor leading to the smaller degree of inclination of equation (1) as compared with the case in which the effective radius of the retard roller 4 is fixed.
It can be seen from this that, in the conventional sheet feeding apparatus, the contact pressure of the retard roller 4 for the feed roller 3 fluctuates upon drive input to the pair of sheet feeding rollers 3 and 4.
The sheet S, fed as described above, undergoes image forming processes in the copying machine, such as development, transfer, and fixing before it is discharged.
Note that, in the above-described conventional technique, in which the contact pressure of the retard roller 4 for the feed roller 3 fluctuates upon drive input to the pair of sheet feeding rollers 3 and 4, it can happen, in actuality, that the feeding area shown in FIG. 7 is greatly departed from, which leads to a problem from the viewpoint of a stable sheet feeding condition. For example, assuming that the feeding condition is determined by the point N1 shown in FIG. 7, a change in fluctuation pressure for positive results in the “double feeding area” being entered, with the result that the feeding area is departed from.
Further, while in FIGS. 9 and 10 the fluctuation pressure Na expressed by equation (1) is given, using the angle θ of the pivot 102A of the pressurizing arm 102 as a parameter, it is to be noted that a large degree of inclination of equation (1) means a wide range of fluctuation in the fluctuation pressure Na with respect to the change amount of the angle of pivot of the pressurizing arm 102. Thus, when the influence of variation in the change of the angle due to dimensional tolerance is taken into account, a large degree of inclination of equation (1) leads to a large degree of variation in the fluctuation pressure Na, which means the sheet feeding latitude is so much the less.